The present invention relates to a highly efficient compact smelting system which uses fine coal as the source of process energy and as the source of carbon for molten metal carburization. The motive force for circulation of liquid metal through separated treatment chambers is provided by the coal or the injection of gases or other gas generating solids. Provision is made for separation of the three key process functions of smelting reduction, melt carburization, and heating to permit optimal process configurations for site specific applications.
An integrated metallurgical reactor is described in my co-pending application, Ser. No. 07/400,566, filed Aug. 30, 1989. The integrated metallurgical reactor utilizes a central carburization chamber and a channel that circumferentially surrounds this chamber and provides locations for continued smelting, and refining in a very compact arrangement. As part of the device, the concept was introduced of injecting coal into a riser channel that was the lower portion of the combustion-carburizing chamber to provide an uplift action which not only injected coal fines for carburization but provided uplift energy for recirculating molten materials from the smelting channel back into the carburizing chamber. The functions of carburizing, and smelting reducing, and refining were separated and the off gases could be separately controlled, but the heating and carburizing were combined in a single chamber. The compact smelting reactor shown therein provided many advantages over existing devices. The present invention represents an improved process for a further separation of heating and carburization functions and for even more efficient use of input energy.
Also, the prior art discussed in my aboveidentified application outlined the state of the art in smelting technology. One recent approach to direct smelting processes is shown in U.S. Pat. No. 4,701,217, issued Oct. 20, 1987, where two large furnaces of a geometric configuration similar to those employed in nonferrous reverberatory furnace practice are operated side by side. One of the furnaces is used for processing lump coal and the combustible gases produced by the volatilization of the lump coal flow into the second furnace where smelting and heating take place. In the second furnace carbon monoxide gases generated by high temperature smelting reduction of iron oxide feedstocks combine with the coal volatiles and flow to the combustion section of the furnace where the mixture is combusted with preheated air and/or oxygen introduced through overhead lances in a manner similar to that employed in reverberatory furnace practice. This device utilizes two large furnaces, and achieves a high degree of heat transfer from the combustion gases to the metal by providing for a large clean metal surface. However, this requires a correspondingly large furnace hearth area and to avoid hearth refractory problems the flowing metal is supported on a large pool of molten lead. The presence of such a large reservoir of molten lead at a temperature in excess of 1300.degree. C. poses serious potential environmental problems. The side-by-side placement of the furnaces consumes a substantial amount of space, and leads to inefficiencies in the transfer of materials into the various stages in the two furnaces and high heat losses from the large surface area of the system.
Another approach, shown in a paper presented by Innes et al. at the Iron & Steel Society meeting, Toronto, April 1988, is based on the "in-bath" principle in which coal and iron ore feed materials are injected into a molten metal bath. The off gases are combusted above the melt by air or oxygen introduced through a post combustion lance and a portion of the heat of combustion is transferred back into the molten metal. A single vessel is thus employed for coal processing, ore smelting and heating. This system is known as the HIsmelt process.
The overall process efficiency in the prior art depends on the degree of post combustion and the heat transfer efficiency from the combustion gases to the melt. An inherent inefficiency of these systems is the inability to prevent some contact between oxidizing gases such as oxygen or carbon dioxide and the molten metal. By this mechanism, carbon in the melt is oxidized to carbon monoxide and the degree of post combustion is reduced.
As a result of the various limitations discussed above, these processes are likely to find only limited industrial application. However, the present invention relies on process optimization by the clear separation of the fundamental process functions and permits a range of engineering solutions and operating strategies. For instance, in one case it may be desirable to combine carburization and heating as in my earlier disclosure. Alternatively, it may be desirable to combine the smelting and carburization as described hereinafter. Finally, it may be preferred to keep all of the functions separate, for example when separate use of the process gases may be advantageous.